A hearing aid comprises an input transducer, an amplifier and a receiver unit. When sound is emitted from the speaker of the receiver unit some of the sound will return to the input transducer. This sound that returns back to the input transducer will then be added to the input transducer signal and amplified again. This process may thus be self-perpetuating and may even lead to whistling when the gain of the hearing aid is high. This whistling problem has been known for many years and in the standard literature on hearing aids it is commonly referred to as feedback, ringing, howling or oscillation.
Feedback thus limits the maximum stable gain that is achievable in a hearing aid. Some traditional approaches to avoid this feedback problem utilizes a feedback cancellation unit by which the feedback path is adaptively estimated and a feedback cancelling signal is generated and subtracted from the input signal to the hearing aid. Hereby as much as 10 dB additional gain is achievable before the onset of whistling.
However, even in very good adaptive digital feedback cancellation systems for hearing aids there will always be a residual error, e.g. the gain of the feedback cancellation signal will either be too large, in which case the feedback is overcompensated to such an extent that the hearing aid gain will not be adequate, or too small, in which case the gain of the signal will exceed the maximum stable gain limit and whistling may occur.